In power train design, it is often desirable and even necessary to connect a driving member to a driven member for joint rotation when the two axis of rotation are misaligned. While the use of a simple Cardan (or Hooke) universal joint would connect the two for joint rotation, this joint will induce angular velocity variations in the driven member, causing vibration at a frequency of twice the rotational velocity and oscillation in the torque transferred to the driven member.
In an effort to provide for more uniform power transmission, constant velocity joints have been developed. Frequently, such constant velocity joints require some sliding motion between elements in the joint, generating friction and power loss. However, some designs, including the one disclosed in U.S. Pat. No. 4,208,889, employ a number of elastomeric bearings to provide for movement in the constant velocity joint.
One specific application for the concept of the constant velocity joint is use in the drive train of a helicopter employing a flapping yoke. In such an environment, a mast connected to the power source rotates about a fixed axis relative to the helicopter main frame. A hub assembly is mounted to the mast for joint rotation. The hub assembly includes a yoke and a torque transferring mechanism to transmit torque from the mast to the yoke. The yoke is also supported by the torque transferring mechanism for flapping motion where the rotational axes of the mast and yoke can become misaligned. U.S. Pat. No. 4,323,332, issued Apr. 6, 1982, discloses an effort to provide a flexible connection between a mast and yoke. U.S. Pat. No. 4,477,225, issued Oct. 16, 1984, discloses an attempt to provide an elastomeric mounting of a yoke to a mast with a constant velocity joint.
In the particular environment of a helicopter, certain factors are of critical interest. The use of a constant velocity joint for the torque transferring mechanism is advantageous in avoiding vibration that would be induced by employing a conventional Cardan joint. In addition, the joint should have a high torque transmitting capacity and preferrably require no lubrication. The joint should be self-centering in order to influence the yoke to return to alignment with the mast after flapping. If the joint should fail in service, the failure is preferably in a noncatastrophic failure mode which could permit the hub assembly to remain operational until the helicopter can be landed. It is also very advantageous to minimize the weight and size of the joint, thereby increasing the useful payload of the helicopter and improving the aerodynamic configuration as well. Finally, some axial motion between the yoke and mast is common and the joint should be capable of transmitting torque despite this motion. In other environments, a bidirectional drive feature, i.e. where either member could be the driving member, can be advantageous.
At this time, no constant velocity joint has been developed which satisfies the above criteria to an adequate degree. Therefore, a need exists for development of a constant velocity joint which satisfies these requirements, and in particular for a constant velocity joint which is adaptable for use in a flapping yoke helicopter environment.